Ballistic light is defined as the light which traverses a scattering medium in the same direction as the incident light. Conventionally, ballistic propagation is pictured as photons which are undeflected in transmission. Such a picture, henceforth called the photonic model, is extensively used in optical tomography, and it explains many properties of ballistic propagation. For example, the photonic model explains the emergence of ballistic light from a thick turbid medium at an earlier time than the scattered light. However, this model is incomplete, as the wave nature of the light is not considered.
Interferometers have been used to measure phase changes based on fluctuations in path length. Phase measurements using interference microscopes, for example, have been used previously to provide two dimensional images of thin tissue samples.
However, there is a continuing need for improvements in systems and methods for measuring turbid media such as tissue.
The phase velocity of light traversing a diffuse scattering medium is a function of scatterer size. To measure this effect optically, an interferometer that measures very small differences in phase velocity between at least two harmonically related wavelengths is used, such as 800 and 400 nm, for example. One wavelength that is an integer multiple of the other wavelength can thus be used to provide quantitative phase information regarding a scanned region of interest. A pair of such wavelengths can be generated harmonically or by using two separate light sources which satisfy the integer multiple requirement to within 5% of the lowest wavelength, i.e. one wavelength is about an integer multiple of the other wavelength. In a preferred embodiment, the interferometer system of the present invention is sensitive to phase velocity differences at least of 40 m/s in a 2 cm thick turbid sample, for example, or equivalently an optical path length difference of about 5 nm. This sensitivity provides for the measurement of very dilute turbid media, a more relevant model for optical applications such as biomedical imaging and remote sensing through atmospheric conditions such as smoke or fog.
The variations in phase velocity result from the wave nature of ballistic propagation and can be measured by treating the ballistic electromagnetic field as the interference of the input light field with the scattered field. Using van de Hulst and Mie scattering theories, ballistic propagation separates into three regimes: (1) When the scatterer size (xcex1) is much smaller than the optical wavelength (xcex), the turbid medium may be approximated as a bulk medium for phase velocity considerations; (2) when a is comparable to xcex, the phase velocity is strongly dependent on scatterer size; (3) when xcex1 is much larger than xcex, turbidity can be ignored for phase velocity considerations. Consequently, by measuring tissue with appropriate harmonically related wavelengths of light, the size and distribution of cellular structures within the tissue can be measured.
Ballistic light can propagate with a phase velocity that is uncharacteristic of the constituent materials of the turbid medium. Hence, the ballistic light itself must carry phase information about the structure and composition of the turbid medium. The photonic model simply cannot explain this variation in phase velocity.
A preferred embodiment of the invention relates to a microscopy imaging system referred to herein as phase dispersion microscopy (PDM). This system is based on measuring the phase difference between a fundamental wavelength of light and a harmonic of unscattered light that are transmitted through a medium. PDM employs an interferometer that substantially reduces or eliminates noise due to optical path length fluctuations. In other phase measurement techniques, it is difficult to account for minute interferometer path length differences in the measured phase. Thus, without an independent way of eliminating such jitter, phase measurements cannot directly yield physically relevant information. In contrast, the phase measured in the present system is independent of path length errors. As an example, the system is used to measure very small anomalous phase velocity differences experienced by ballistic light during propagation through turbid media. The present system and method can provide quantitative information by measuring the refractive index dispersion of very dilute material such as DNA-water solutions. The sensitivity of the technique and its image formation capabilities can be applied of the imaging of an unsustained tissue section.
This technique can be used to provide two dimensional (2D) or three dimensional (3D) imaging of tissue both in vitro and in vivo. Additional details regarding the systems and methods of the invention can be found in Application Ser. No. 60/200,187 filed on Apr. 28, 2000 which is incorporated herein by reference.